1. Field of the Invention
The present invention relates to an electrophotographic type color image forming apparatus such as a printer, a copying machine or the like, and particularly to a structure for cooling light-scanning units in a color image forming apparatus, arranged in a so-called tandem in which a plurality of light-scanning units for respective colors are sequentially arranged along a conveying path of recording sheets of paper.
2. Description of the Related Art
In a digital color copying machine, for example, after image processing is performed on respective color separation images of an original document which has been read by a scanner and input from the scanner, images are formed on photosensitive bodies by image forming means having a light-scanning unit provided for respective colors and are superimposed on a recording sheet of paper to form one color image. In order to form an image for each color, each image forming means includes a photosensitive body on which an electrostatic latent image is formed, a charging device for charging the photosensitive body, a light-scanning unit for exposing a surface of the charged photosensitive body to form an electrostatic latent image, a developing means for attaching toner onto the electrostatic latent image which has been formed on the photosensitive body to visualize the latent image, and the like. A recording sheet of paper with a color image obtained by superimposing the developed images on the recording sheet of paper is then conveyed to a thermally fixing or fusing means, where toner forming a visualized or color image is fused or melted onto the recording sheet of paper. Thereafter the recording sheet of paper is ejected outside the machine body.
In such a digital color copying machine, images of respective colors are reproduced faithfully and the images of the respective colors are superimposed on a recording sheet of paper with a high accuracy. This is an image representation included in an original image can be accurately reproduced. Recently, however, in order to minimize a space for installing an apparatus, compact-sizing and downsizing of an apparatus are needed. As a result, a drawback or problem has developed where constituting elements inside the apparatus are isolated from one another, and also interfere with one another. Particularly, when the interior of the apparatus is elevated to a high temperature, distortion, flexure, torsion and the like occur due to thermal expansions of respective elements in an optical device portion, such as a light-scanning unit or the like, and the image to be recorded is this adversely influenced in an image-writing step.
In view of the above, an apparatus has been developed and structured such that air flow is generated between image forming means (which is a processing portion for recording and reproducing an image) and thermally fixing means generating the highest heat in an apparatus, so that heat generated from the thermally fixing means does not influence the processing portion. In Japanese Unexamined Patent Publication JP-A 3-56980 (1991), partition walls are provided among a photosensitive body drum unit, an optical system unit and a fusing or fixing unit, and respective airs in spaces isolated by the partition walls are sucked and exhausted by one fan, thereby making it unnecessary to provide a duct. Thus, a air flow path is formed as a duct and heat atmosphere about heat sources is exhausted forcibly, thereby suppressing increase in temperatures of peripheral devices. Similarly, in Japanese Unexamined Patent Publication JP-A 4-257880 (1992), also, an upper portion of thermally fixing means and a photosensitive body module are cooled by one fan.
In these prior art apparatus, however, there is no consideration or disclosure related to a digital color copying machine this is provided with a plurality of light-scanning units for respective colors, and with a plurality of image forming means.
In a digital color reproducing machine, when the respective light-scanning units are influenced by the heat of a thermally fixing or fusing means, images of respective colors are not faithfully reproduced respectively. Thus a color shear occurs even when the respective images are accurately superimposed on a recording paper, thereby injuring or deteriorating the image quality of a color image formed on the recording sheet of paper. As a result, in an image forming apparatus where a plurality of light-scanning units are arranged in tandem along a conveying path of recording sheets of paper, there is still room for improvement, so that influence due to heat generated by the fixing means is suppressed to a low level and a high image quality of an image is obtained.
Also, in the conventional art, in order to reduce influence due to heat, when a exhausting flow rate of air in the periphery of the thermally fixing means is increased, temperature lowering within the thermally fixing means itself may conversely occur. In the worst case, since there occurs a problem such as poor fixing or the like due to insufficient fixing temperature would occur, there is a further limitation in a structure where cooling depends on only cooling the periphery of the thermally fixing means.